1. Field of the Invention
The present invention relates to a low frequency responsive electromechanical transducer, operable as an accelerometer, a velocity transducer or as a displacement transducer.
2. Description of the Prior Art
Seismically mounted motion sensors are available. See U.S. Pat. Nos. 2,754,435 and 3,157,852. Such devices normally operate over ranges which are well above the natural resonance frequency of their spring-mass system. Hence at low frequencies the seismic sensors lose sensitivity. Accelerometers are available for sensing motion at frequencies below the natural frequency of their spring-mass system.
There are numerous accelerometer devices available for measuring the instantaneous acceleration applied to a device when the accelerometer is exposed to high frequency cyclic movement. Where low frequency cyclic movement is encountered, there are available suitable devices for measuring the instantaneous acceleration. One type of low frequency device is a strain gauge, piezoresistive type, which has a shortcoming in that its sensitivity is poor, to wit, its sensitivity is in the range of 10 to 20 millivolts/g. Such devices can be made more sensitive but they become quite fragile. The other type of low frequency device is a servo accelerometer sometimes called a forced balance accelerometer or an inertial guidance accelerometer. This device has a high sensitivity and operates in the range of 0 to 15 Hertz. Its principal shortcoming is its high initial cost.
There exists a need in the art for a device which will accurately detect acceleration of cyclic movement at low frequencies with a high degree of sensitivity and which can be produced for a relatively modest cost.
Eddy current type displacement sensing devices are well known in the art as devices which can be used to measure the instantaneous displacement between themselves and a metallic target within their measuring zone. See U.S. Pat. Nos. 3,521,158; 3,521,159; 3,541,394; 3,707,671.